The present invention generally relates to a rotary head type digital signal recording and reproducing apparatus, and in particular to a rotary head type ditital signal recording and reproducing apparatus known as R-DAT (rotary head type digital audio tape recorder) conforming to industry standards for the same, in which by use of rotary heads alternately scanning a magnetic tape, a tracking reference signal and a sub-channel signal are recorded on and reproduced from predetermined starting and ending portions of each track, while a digital audio signal is recorded on and reproduced from an intermediate track portion of each track other than the above track portions.
In a digital audio tape (DAT) recorder, an analog audio signal is modulated into PCM audio data by a pulse code modulation (PCM), and the PCM audio data are recorded on and reproduced from a magnetic tape. In a rotary head type digital audio tape recorder which employs rotary magnetic heads, data are successively recorded on and reproduced from tracks formed obliquely to a longitudinal direction of the magnetic tape without a guard band between two mutually adjacent tracks, alternately by a pair of rotary heads having gaps of mutually different azimutyh angles.
A signal format of each track formed on the magnetic tape has a PCM audio data region, a pair of tracking reference signal regions and a pair of sub-channel (sub-code) signal regions. The PCM audio signal region is allocated in an intermediate portion of one track and records the PCM audio signal. The pair of the tracking reference signal regions are positioned on both sides of the PCM audio signal region. Likewise, the pair of sub-channel signal regions are positioned on both the sides of the PCM audio signal recording region. The pair of the tracking reference signal regions record the tracking reference signal. The pair of the sub-channel signal regions record a program number, a time code and the like.
Between mutually adjacent signal recording regions, there are provided inter-block gaps (IBGs). Therefore, re-recording is obtainable by separately over-writing the PCM audio data and the sub-code signal in the corresponding regions. For example, while reproducing the magnetic tape in which the PCM audio data are already recorded and monitoring a corresponding reproduced audio signal, it is possible to record on the magnetic tape the sub-code signal including a flag which indicates a program starting position relating to the monitored content, a program number indicating an order of music recorded, a program time, a table of contents (TOC) and the like. This kind of recording is called an after-recording.
At the time of the after-recording, the rotary heads successively perform the recording of the sub-code signal and the reproduction of the PCM audio data in accordance with a predetermined time allocation.
The PCM audio data are recorded and reproduced in at least two kinds of modes. In a standard tape speed mode (hereafter referred to as a standard mode) of the R-DAT standards, the PCM audio data have a sampling frequency of 48 kHz, two channels and a linear quantization of sixteen bits. On the other hand, in a half tape speed mode (hereafter referred to as a half-speed mode), the PCM audio data have a sampling frequency of 32 kHz, two channels and a non-linear quantization of twelve bits. Actually, there are other non-linear modes such as a mode in which the PCM audio data have a sampling frequency of 44.1 kHz, four channels and a quantization number of twelve bits, however, these other modes all have the same tape speed as the standard mode.
In the half-speed mode, the rotational speed of a rotary drum on which the rotary heads are mounted and the tape transport speed are respectively set to speeds which are one-half those in the standard mode. In addition, the frequency of digital signals (more accurately, the frequencies of clock pulses for producing the PCM audio data and the tracking reference signal) are set to one-half those in the standsrd mode. In other words, the operation speed of the digital audio tape recorder as a whole in the half-speed mode is set to one-half that in the standard mode, except for a part of the digital audio tape recorder where a conversion is carried out between the analog audio signal and the digital signal.
The data rate in the standard mode is 48 (kHz).times.2.times.16=1536 (kbits/sec), and the data rate in the half-speed mode is 32 (kHz).times.2.times.12=768 (kbits/sec). Accordingly, the sound quality obtained in the half-speed mode is slightly deteriorated when copared to that obtained in the standard mode, but there is an advantage in that the play time in the half-speed mode is twice that in the standard mode for a given length of a magnetic tape because the operation speed of the digital audio tape recorder in the half-speed mode is set to one-half that in the standard mode.
It is desirable that the half-speed mode is added to the rotary head type digital audio tape recorder having the standard mode and, in addition, that the after-recording can be satisfactory performed even in the half-speed mode. However, due to the following problems, the realization of such a digital audio tape recorder is costly and technically difficult, and would cause deterioration in the quality of the digital audio tape recorder.
First, in the after-recording at the time of the half-speed mode, the sub-code signal is recorded with a time twice that in the standard mode. For this reason, a long time is necessary for the after-recording.
Secondly, the tracking refernce signal frequency and the carrier frequency of the PCM audio data in the half-speed mode become one-half those in the standard mode. For this reason, particularly in the rproducing mode, the operation frequencies of an analog filter circuit part and a phase locked loop (PLL) circuit part for reading data within a signal processing circuit for processing the tracking reference signal and the PCM audio data must be switched between the standard and half-speed modes. Alternatively, it is necessary to provide a circuit part exclusively for use in the standard mode and another circuit part specifically for use in the half-speed mode.
Thirdly, the coupling between the rotary heads mounted on the rotary drum and recording and reproducing amplifiers is normally made through a rotary transformer. However, since the signal frequency in the half-speed mode becomes one-half that in the standard mode, the coupling in the low frequency range becomes deteriorated in the half-speed mode.
Fourthly, when the setting is made so that an optimum carrier-to-noise ratio is obtained in the standard mode, an output voltage of the reproducing rotary head in the half-speed mode becaomes one-half that in the standard mode because the relative linear velocity between the magnetic tape and the rotary head in the half-speed mode is one-half that in the standard mode. As a result of this setting, the carrier-to-noise ratio becomes deteriorated in the half-speed mode.
Fifthly, a drum motor for rotating the rotary drum and a capstan motor for rotating a capstan which drives the magnetic tape must have predetermined performances in the two rotational speeds corresponding to the standard and half-speed modes, where the predetermined performances refer to the tolerable range of the jitter in the rotation of the rotary drum for maintaining phase synchronism between the rotation of the rotary drum and an electrical circuit, the tolerable range of instability of the rotation of the capstan motor, and the like.